Science 9: Unit B

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Science 9 Unit B

• Topic 6 Chemical Compounds

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What is a compound?

• A compound is when two or more element atoms
  combine together by either giving up electrons or
  sharing them. A compound is usually much more
  stable than the individual elements by
  themselves.
• There are two kinds of chemical compounds ionic
  and molecular.

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Ionic Compounds

• 1. Ionic Compounds - form when a metal atom gives
  up one or more valence electrons to a nonmetal
  atom. The two atoms become oppositely charged and
  therefore become bonded to each other. This is
  called an ionic bond.

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Molecular Compounds

• 2. Molecular Compounds form when two or more
  non-metal atoms share their valence electrons.
  The bonds in a molecular compound are weaker than
  in an ionic compound so the compound is not as
  stable as an ionic compound.

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Molecular Compounds

• A molecule is the smallest unit of a molecular
  compound. A molecular compound is made up of many
  molecules bonded to each other. Eg. Water is made
  up of many H2O molecules bonded together.
• The bond between molecules is weaker than the
  bond within the molecule. That is why it takes
  less energy to split a piece of wood than to burn
  wood.
• Molecular compounds, unlike ionic compounds are
  poor conductors of heat and electricity and are
  usually gases and liquids instead of solids.

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Naming Molecular Compounds

• II. Naming Molecular Compounds
• Write the name of the first element
• Write the name of the second element, but change
  the ending (after the first syllable) to ide. So
  oxygen would be oxide, chlorine would be
  chloride, hydrogen be hydride, etc.
• Write the correct Greek prefix in front of each
  element name to indicate how many atoms of that
  element there are.
• Do not use the prefix mono- for the first element
  name, just write the element name as is.

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Molecular Compound Naming System The Greek
Prefixes
Number Greek Prefix
1 atom Mono
2 atoms Di
3 atoms Tri
4 atoms Tetra
5 atoms Penta
6 atoms Hexa
7 atoms Hepta

• For example H2O dihydrogen monoxide (di
  because there are 2 hydrogen atoms and mono
  because theres just the one oxygen atom). And
  NH3 nitrogen trihydride (NOT mononitrogen
  trihydride).

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Ionic Compounds - Naming

• Ionic compounds form when a metal gives up an
  electron to a nonmetal atom. Now the metal is
  positively charged and the nonmetal is negatively
  charged. Since they have opposite charges they
  bond together and form a stable ionic compound.
• There is no sharing of electrons here, the metal
  actually gives up its electrons to the nonmetal.
• When an atom gives or takes in extra electrons we
  call it an ion. Metal ions (positive charge) are
  called cations, nonmetals ions (negative charge)
  are called anions.

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Ionic Compounds Contd

• A common example of an ionic compound is table
  salt NaCl(s). Sodium ions give up one electron
  to a chlorine atom to form an ionic compound, now
  they both are more stable noble-like atoms.
• Since ions have electric charges they can conduct
  electricity this property is called conductivity.
  Molecular compounds usually do not have this
  property so an easy way to tell them apart is to
  see if they conduct electricity or not.

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Naming Ionic Compounds

• Step 1 Write name of metal first
• Step 2 Write name of nonmetal second and change
  ending to ide.
• Step 3 For transition metals (metals in the
  middle area of the periodic table), you must
  write down the roman numeral (in brackets) for
  the ion charge of the cation. Eg.
  iron(II)chloride is a different compound than
  iron(III)chloride.

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Odds and Ends

• Diatomic Compounds Special kind of molecular
  compound where two of the same non-metals
  elements will form a molecular bond together and
  share their electrons. Eg. oxygen gas is O2 .
  HONI Brushed Clean Fingers is mnemonic device to
  help you remember which elements form diatomic
  compounds.
• Because atoms in molecular compounds are sharing
  electrons they are not as stable as atoms in
  ionic compounds which actually give up or take in
  extra electrons. Sharing electrons means that
  some of the time the electrons are in another
  atom, this makes the atom unstable during that
  time. In an ionic compound all atoms are stable
  all the time.
• The stability of ionic compounds is shown when
  you try to perform reactions with them. You can
  split water into hydrogen and oxygen (chemical
  reaction) with a small electric current, but it
  takes 900C just to melt a typical ionic compound
  like NaCl(s) (a physical change).

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